1. Field of the Invention
The present invention relates to an apparatus and method for performing GMSK (GAUSSIAN MINIMUM SHIFT KEYING) baseband modulation, and particularly to an apparatus and method for building and storing a plurality of parameters and their corresponding tabular data which is simplified previously and using the tabular data to rapidly and correctly modulate input signals.
2. Description of the Related Art
GMSK baseband modulation is widespreadly used in digital wireless baseband modulating technology. Because the amplitude of the signal after modulation is constant, the situation of a nonlinear distortion will not occur, and thus a nonlinear (class C) RF power amplifier is suitable for better performance. The prior design flow of implementing GMSK baseband modulator is described hereinafter. Firstly, a digital filter generates a frequency track according to the input signals. Secondly, an accumulator computes phase shift of modulating signals. Finally, calculate a set of in-phase signals In and a set of quadrature-phase signals Qn based on a look-up table stored in a memory in which sine and cosine mathematical functions are listed in the table. The above-mentioned technology is shown in xe2x80x9cA 270 Kb/s 35-mW Modulator IC for GSM Cellular Radio Hand Held Terminals,xe2x80x9d by J. Haspeslaghet al., IEEE J. Solid-State Circuits, Vol. 25, pp.1450-1457, December 1990. The prior art method has some disadvantages. For instance, a accumulator inside the prior art structure would create an accumulation error and a quantization error, and a lot of memories are needed for storing mathematical functions representing phase curves. Another prior art method is shown in xe2x80x9cEfficient Implementation of an I-Q GMSK Modulator, by Alfredo Linz and Alan Hendrickson,xe2x80x9d IEEE Transaction on Circuit and System, Vol. 43, No. 1, pp. 14-23, January 1996. In this book, a frequency offset is integrated to get the difference between phases, and uses the resulting output phase curves during a period of transferring two symbol bits to approximate all possible output signals. This prior art method is only suitable for the circumstance of 3-bit input signals. When the bit number of input signals is over three, the design complexity will too rapidly increase to implement easily. Besides, if only using three bits as indexes to look up the tables in memory, the accumulation error will exceed a threshold of the system, particularly in the case of the smaller parameters like BT (the product of the lower 3 dB bandwidth of a low pass Gaussian filter multiplied by the time needed for transmitting one-bit digital data). As mentioned above, the desired performance of the GMSK baseband modulation: rapidity, low distortion and lower hardware cost, are not completely achieved by the prior art.
Accordingly, the object of the present invention is to resolve the following drawbacks: (1) the conventional GMSK baseband modulation uses complex mathematical operations to obtain the in-phase signal I(t) and the quadrature-phase signal Q(t), and (2) the parameters of conventional GMSK baseband modulation can not be adjusted as desired. In order to accomplish the object, the present invention proposes an apparatus and method for performing GMSK baseband modulation based on a reference phase to be simplified. The present invention simplifies the definition and formula of the in-phase signal I(t) and quadrature-phase signal Q(t) in advance, and stores results and corresponding parameters into look-up tables inside a memory. Thus, the present invention only needs a simple logic and control circuit to find out the corresponding data in the memory, and calculates digital in-phase signal In and digital quadrature-phase signal Qn through simple operations. Besides, the important parameters of the present invention, such as BT, can be adjusted by the user to change the operating characteristic of the system. The present invention only needs small memory capacity, a little operation by the whole circuit, and a little operation time in computation. Therefore, the present invention can transmit data at a high speed. Furthermore, because various results are accurately calculated by the present invention in advance, the accumulation errors will be largely reduced. In conclusion, the present invention reduces the time of operations and memory space, and prevents the accumulation errors occurring easily in the conventional GMSK baseband modulation.
The present invention in baseband modulating apparatus mainly comprises a memory unit and a processing unit. The memory unit which stores a plurality of parameters and their corresponding tabular data of a low pass Gaussian filter, the parameters and their corresponding tabular data being simplified in advance; the low pass filter being used for reducing the bandwidth of the apparatus and increasing the utilization of the bandwidth; and a processing unit based on the input digital data and the tabular data in the memory unit to generate the digital in-phase signal In, and the digital quadrature-phase signal Qn. The processing unit comprises: an input device for recording the input digital data; an accumulator connected to the input device to generate a reference phase xcex8ref; and a control unit, connected to the input device, the accumulator and the memory unit, for generating control signals and address access signals associated with the memory unit.
The present invention in baseband modulating method mainly comprising: the step of building and storing a plurality of parameters and their corresponding tabular data of a low pass filter, the parameters and their corresponding tabular data being simplified in advance; the low pass filter being used for reducing the bandwidth needed and increasing the utilization of the bandwidth; the step of sequentially recording the input digital data; the step of determining a reference phase xcex8ref; and the step of receiving the tabular data which is associated with the input digital data; and the step of generating the digital in-phase signal In and the digital quadrature-phase signal Qn by using the reference phase and the received tabular data
The present structure can also be implemented by software, because of simplicity and less operations of the structure, the implementation by software also has the advantage as above-mentioned.